This invention relates to a thin resistor film type thermal head used in printing on heat-sensitive paper.
A thermal head for printing on heat-sensitive paper comprises a large number of thin minute heat-generating resistor films arranged linearly or in the matrix form. Desired printing on heat-sensitive paper is effected by selectively introducing current through the heat-generating resistor films thus arranged. Known thermal heads for printing on heat-sensitive paper include not only a thin resistor film type but also a thick resistor film type and a semiconductor resistor type.
Generally, a thin resistor film type thermal head has the advantages over a thick resistor film type that heat-generating resistor dots can be arranged with a higher density, more saves power consumption, makes a quicker response to heat, and carries out printing more speedily. However, the thin resistor film type thermal head raises problems in respect of manufacturing cost and power withstanding characteristic.
For a distinct printed image, it is necessary to reduce the size of heat-generating dots in order to arrange them with a higher density. At present, however, a thin resistor film type thermal head is often constructed by arranging heat-generating resistor dots at the rate of 5 to 8 dots per millimeter in consideration of various factors such as problems in manufacturing technique, driving circuits and electrical connection therewith. Heat-generating resistor dots used at present mostly have a square shape or a rectangular shape whose longitudinal and crosswise sides bear the ratio of 2:1 at maximum.
Power of about 0.1 to 0.5W is required per resistor to cause a plurality of heat-generating resistors jointly to produce a sufficient amount of heat for coloration of heat-sensitive paper. The higher the density of resistor dots, the smaller the area thereof. Accordingly, the resistor dots increase in power density with the resultant decline in power withstanding characteristic.
The resistance of the respective heat-generating resistors is subject to certain limitations by different factors such as driving circuit and power source, and has a considerably wide range of variation, depending on the design of, for example, the driving circuit. However, heat-generating resistors in general use have a resistance of scores of or hundreds of ohms. If falling in resistance, the heat-generating resistor increases in current capacity, raising, for example, the problem that the resistance of a lead and circuit, etc. can not be overlooked. Conversely, where the heat-generating resistor unduly rises in resistance, the thermal head tends to be impressed with undesirably high voltage.
Hitherto, a thin resistor film used with a thermal head has generally been of a Ta--N type mainly consisting of tantalum and nitrogen. This Ta--N type resistor film is prepared by sputtering tantalum in an atmosphere of argon mixed with nitrogen. The larger the content of nitrogen, the higher the resistance of the resistor film. However, a heat-generating resistor has generally been formed of a Ta.sub.2 N system having good reliability and stability.
This Ta.sub.2 N system has a specific resistance of about 200.mu..OMEGA.-cm. For increased resistance, the Ta.sub.2 N type heat-generating resistor film has to be made thin and be shaped into the form of an elongate dot. However, manufacture of the Ta.sub.2 N type resistor film is subject to certain technical limitations. Namely, the Ta.sub.2 N type resistor film should be made as thin as hundreds of A units in order to have a dot shape and a desired degree of resistance. For example, the Ta.sub.2 N type resistor film should take an elongate dot form whose longitudinal and crosswise sides bear a ratio of 2:1, and also have a thickness of about 200A in order to have a resistance of 200 ohms. However, the formation of such a thin Ta.sub.2 N resistor film is accompanied with various problems, for example, that difficulties arise in controlling the Ta.sub.2 N resistor film to be made uniformly thin to the above-mentioned extent; the reproducibility and uniformity of resistance decrease; and elevated current density would likely lead to the destruction of the Ta.sub.2 N resistor film.
For elimination of the above drawbacks, an attempt has been proposed to convert an ordinary square heat-generating resistor 1 into a linear form as illustrated in FIGS. 1a and 1b. In these FIGS. 1a and 1b, reference numeral 2 indicates individual electrodes and reference numeral 3, common electrode. These constructions have been devised as an attempt to increase the resistance of a heat-generating resistor film without unduly reducing its thickness. However, this attempt has the drawback that the required greater density and higher resistance of thin dot-shaped resistor films demand a more advanced patterning technique, leading to a decline in yield.